An exhaust system for releasing exhaust gas from an automobile engine to the outside environment is explained below. FIG. 8(a) shows a cutaway perspective view of an exhaust system connected to an engine of a conventional or existing automobile.
As shown in FIG. 8(a), an exhaust system A includes: an exhaust manifold M1 for collecting exhaust gas from cylinders of an engine E; a flexible tube P1 connected to the exhaust manifold M1; a catalytic converter C for purifying toxic components in the exhaust gas by oxidation with catalysts; a center pipe P2; a muffler M3 for attenuating the throbbing of the engine E utilizing the principles of sequential expansion of the exhaust gas, resonance, interference and absorbance of sonic waves, cooling of the gas and the like; and a tail pipe P3. Depending on types of automobiles, a sub muffler M2 (not shown) may be additionally disposed upstream of the muffler M3.
FIG. 8(b) shows a schematic diagram of (a), and FIG. 8(c) is a diagram in which the flexible tube P1 is placed downstream of the catalytic converter C.
As for conventional flexible tubes, those disclosed in patent documents have been known (for example, see Japanese Kokai unexamined patent publication No. 9-268913). FIG. 9 shows a half sectional view of a conventional flexible tube. As shown in FIG. 9, a flexible tube 80 is disposed between the engine E and the catalytic converter C on the exhaust path from the engine E. The flexible tube 80 includes: an end pipe 82 which is an inlet to be connected to the exhaust path from the engine E; an end pipe 83 which is an outlet to be connected to the catalytic converter C: a bellows-shaped tube main body 84 made of metal; and a heat guard tube 85.
Both end parts of the tube main body 84 are fit onto the respective end pipes 82 and 83, and welded to the end pipes 82 and 83, together with the corresponding pipe sleeves 86 held on the outer periphery of the end parts of the tube main body 84. Each of the end pipes 82 and 83 has steps so that the part of the end pipe closer to a middle section of the flexible tube has a smaller diameter. Inside the tube main body 84, a heat guard tube 85 is provided in such a manner that it spans the space between the inner end parts of the pipes 82 and 83.
The heat guard tube 85 is formed of cylindrical cloth woven out of carbon fiber, and flexible fiber material is used in order to lessen the harmful effect of heat on the tube main body 84, to attenuate noise and to reduce heat.
However, this fiber cloth has a problem in that high-temperature exhaust gas easily leaks out through space left between the fibers and reaches the outer bellows, having harmful effects on durability, spring property and the like of the outer bellows.
In addition, exhaust sound also passes through space left between the fibers, reaches the outer bellows, and then leaks out from the flexible tube 80, thus increasing exhaust noise.
In the case where the flexible tube 80 is located upstream of the catalytic converter C, exhaust gas of high temperature directly reaches the outer bellows through space left between the fibers. Therefore, during the initial period of engine start-up, the temperature of the exhaust gas becomes lower and the temperature rise of the exhaust gas to be fed into the catalytic converter C takes longer, resulting in poor performance capability to purify the exhaust gas.
It should be noted that the flexible tube P1 according to the conventional art corresponds to what is called an exhaust pipe.
Accordingly, the present invention was made with the view toward solving the above-mentioned problem, and an object thereof is to provide a flexible tube disposed downstream of the exhaust path from the engine, which may have the following advantages: sound insulation effect is improved; the exhaust efficiency is improved to thereby increase the output performance of the engine; the exhaust gas temperature reaching the outer bellows are suppressed to thereby improve durability of the flexible tube itself; and especially in the case of the flexible tube disposed upstream of the catalytic converter, the lowering of the exhaust gas temperature is prevented to thereby speed up the temperature rise of the exhaust gas to be fed into the catalytic converter, so as to promote the activation of the catalysts and improve the capability to purify the exhaust gas.